Fluid heaters are known for heating ultra pure water and other ultra pure fluids with a minimum amount of contamination. Examples of such a heater are disclosed in U.S. Pat. Nos. 3,870,033 and 3,983,361. The prior art heaters are complex, expensive, and only exhibit 80%-85% thermal efficiency. The present invention overcomes the disadvantages associated with the prior art by providing a relatively compact inexpensive heater for heating ultra pure fluids such as ultra pure water with a minimum amount of contamination and which exhibits a thermal efficiency of greater than 95%.